The document summarizes guidelines for acute oral toxicity testing, including the OECD Guideline 425 for using the up-and-down procedure (UDP). Some key points: - Acute toxicity testing involves administering single doses of a substance to determine effects over 14 days and estimate the lethal dose for 50% of animals (LD50). - Guideline 425 describes using the UDP, which minimizes the number of animals. It involves dosing animals sequentially, with higher doses if an animal survives and lower doses if it dies, to estimate the LD50 and observe signs of toxicity. - Testing follows a limit test or main test procedure. The limit test uses up to 5 animals at a high starting dose

1. ACUTE TOXICOLOGY
OECD GUIDELINES
425
Presented by
PURANAM SOUMYA
Y18PHD0417
III-PHARM.D

2. TOXICOLOGY
 Toxicology is the scientific study of adverse
effects that occur in living organisms due to
chemicals.
 It involves observing and reporting
symptoms, mechanisms, detection and
treatments of toxic substances, in particular
relation to the poisoning of humans.
 Toxicity tests are mostly used to examine
specific adverse events such as cancer,
cardiotoxicity and skin/eye irritation

3. ANIMAL TOXICITY TEST
Acute toxicity
Sub-acute (repeated doses) toxicity
Sub-chronic toxicity
Chronic toxicity
Special toxicity
14 days
28 days
3 months
6 months to 2 yrs.
e.g. Carcinogenicity

4. ACUTE ORAL TOXICITY
 Acute toxicity testing is carried out to determine the effect of a single dose on a particular
animal species.
 In acute toxicological testing, the investigational product is administered at different
dose levels, and the effect is observed for 14 days.
 All mortalities caused by the investigational product during the experimental period are recorded
and morphological, biochemical, pathological, and histological changes in the dead animals are
investigated.
 Acute toxicity testing permits the 50% lethal dose (LD50) of the investigational product to be
determined.
 Acute toxicity studies provide information on:
• The potential for acute toxicity in humans;
• An estimate of safe acute doses for humans;
• The potential target organs of toxicity;
• Time course of drug-induced clinical observations;
• The appropriate dosage for multiple-dose toxicity studies; and
• Species differences in toxicity.

5. OECD GUIDELINES FOR ACUTE ORAL TOXICITY
Number TITLE Date of Adoption
420 Fixed dose procedure 17 December 2001
425 Up and down procedure 3 October 2008

6. 425 : Acute oral toxicity – Up and Down Procedure
(UDP)
INTRODUCTION
• The concept of the up-and-down testing approach was first described by Dixon and Mood.
• In 1985, Bruce proposed to use an up-and-down procedure (UDP) for the determination of acute
toxicity of chemicals .
• A study comparing the results obtained with the UDP, the conventional LD50 test and the Fixed
Dose Procedure (FDP,OECDTest Guideline 420) was published in 1995.
• The test procedure described in this Guideline is of value in minimizing the number of animals
required to estimate the acute oral toxicity of a chemical. In addition to the estimation of LD50
and confidence intervals, the test allows the observation of signs of toxicity.

7. • The testing laboratory should consider all available information on the test substance prior to
conducting the study.
• It will include
-Identity and chemical structure of the test substance;
-Physical chemical properties;
-Results of any other in vitro or in vivo toxicity tests on the substance;
-Toxicological data on structurally related substances or similar mixtures;
-Anticipated use(s) of the substance.
• This information is useful to determine the relevance of the test for the protection of human
health and the environment, and will help in the selection of an appropriate starting dose.
• The method permits estimation of an LD50 with a confidence interval and the results allow a
substance to be ranked and classified according to the Globally Harmonised System for the
classification of chemicals which cause acute toxicity.
INITIAL CONSIDERATIONS

8. • Dose starting near 175mg/kg and using half log units (dose progression by
factor 3.2) between doses will procedure best results.
• Test substances, at doses that are known to cause marked pain and distress
due to corrosive or severely irritant actions, need not be administered.
• Moribund animals or animals obviously in pain or showing signs of severe and
enduring distress shall be humanely killed, and are considered in the
interpretation of the test results in the same way as animals that died on test.
• The method would not be practical to use when considerably delayed death
(five days or more) can be expected.

9. PRINCIPLE OF LIMIT TEST
• The LimitTest is a sequential test that uses a maximum of 5 animals.A test dose of 2000, or
exceptionally 5000 mg/kg, may be used.
• The selection of a sequential test plan increases the statistical power and also has been made
to intentionally bias the procedure towards rejection of the limit test for compounds with
LD50s near the limit dose.
PRINCIPLE OF MAIN TEST
• The main test consists of a single ordered dose progression in which animals are dosed, one at a
time, at a minimum of 48-hour intervals.
• The first animal receives a dose a step below the level of the best estimate of the LD50.

10. • If the animal survives, the dose for the next animal is increased by [a factor of]
• 3.2 times the original dose; if it dies, the dose for the next animal is decreased
• by a similar dose progression.
• Each animal should be observed carefully for up to 48 hours before making a
decision on whether and how much to dose the next animal.
• That decision is based on the 48-hour survival pattern of all the animals up to
• that time.
• The LD50 is calculated using the method of maximum likelihood

11. DESCRIPTION OF METHOD
Selection of animal species
• The preferred rodent species is the rat ,normally female rats are used.
• When toxicological or toxicokinetic properties of structurally related chemicals
are studied, males are likely to be more sensitive.
• Criteria for female rats
-They should be nulliparous and non pregnant. -
-Should be between 8 to 12 years old.
-Weight ± 20% of initial mean weight

12. Housing and feeding
• Temperature - 22°C (± 3°C).
• Humidity – 50 – 60% not exceeding 70%
• Lighting should be artificial, the sequence being 12 hours light and 12
hours dark.
• The animals are housed individually.
• For feeding, conventional rodent laboratory diets may be used with an
unlimited supply of drinking water.

13. Preparation of animals
• The animals are randomly selected, marked to permit individual identification, and
kept in their cages for at least 5 days prior to dosing to allow for acclimatization to
laboratory conditions.
• Ensure that animals are available in the appropriate size and age range for
the entire study.
Preparation of doses
• Test substances should be administered in a constant volume by varying the
concentration of the dosing preparation.
• In rodents, the volume should not normally exceed 1 ml/100g of body weight; however
in the case of aqueous solutions, 2 ml/100g body weight can be considered.
• For vehicles other than water, the toxicological characteristics of the vehicle should
be known.

14. PROCEDURE
Administration of Doses
• The test substance is administered in a single dose by gavage using a
stomach tube or a suitable intubation cannula.
• Animals should be fasted prior to dosing (e.g., with the rat, food but not
water should be withheld overnight; with the mouse, food but not water
should be withheld for 3-4 hours).
• The animals should be weighed and the test substance administered and
the dose is calculated according to the body weight.
• After the substance has been administered, food may be withheld for a
further 3-4 hours in rats or 1-2 hours in mice.

15. Limit test and Main Test
• The limit test is primarily used in situations where the experimenter has
information indicating that the test material is likely to be nontoxic, i.e.,
having toxicity below regulatory limit doses.
• There is little or no information about its toxicity, or in which the test
material is expected to be toxic, the main test should be performed.
• Information about the toxicity of the test material can be gained from
knowledge about similar tested compounds or similar tested mixtures or
products, taking into consideration the identity and percentage of
components.

16. Limit Test
Limit Test at 2000 mg/kg
• Dose one animal at the test dose.
• If the animal dies, conduct the main test to determine the LD50.
• If the animal survives, dose four additional animals sequentially so that a total of
five animals are tested. If three animals die, the limit test is terminated and the
main test is performed.
• If an animal unexpectedly dies late in the study, and there are other survivors, it is
appropriate to stop dosing and observe all animals to see if other animals will also
die during a similar observation period.
• The results are evaluated as follows (O=survival, X=death).

17. • The LD50 is less than the test dose (2000 mg/kg) when three or
more animals die.
O XO XX
O OX XX
O XX OX
O XX X
If a third animal dies, conduct the main test.
• Test five animals.The LD50 is greater than the test dose
(2000 mg/kg) when three or more animals survive.
O OO OO
O OO XO
O OO OX
O OO XX
O XOXO
OXOOO/X
O OXXO
OOXOO/X
O XXOO

18. Limit Test at 5000 mg/kg
• Dose one animal at the test dose.
• If the animal dies, conduct the main test to determine the LD50.
• If the animal survives, dose two additional animals. If both animals survive, the
LD50 is greater than the limit dose and the test is terminated.
• The LD50 is less than the test dose (5000 mg/kg) when three or more animals die.
O XO XX
O OX XX
O XX OX
OXXX0.
The LD50 is greater than the test dose (5000 mg/kg) when three or more animals
survive.
OOO
O XOXO
O XOO
O OXXO
O OXO
OXXOO

19. Main Test
• Single animals are dosed in sequence usually at 48 h intervals.
• For selecting the starting dose, information on structurally related
substances and results of any other toxicity tests on the test material, should
be used to approximate the LD50 as well as the slope of the dose- response
curve.
• The dose progression factor should remain constant throughout testing.
• When there is no information on the slope of the substance to be tested, a
dose progression factor of 3.2 is used.
• Using the default progression factor, doses would be selected from the
sequence 1.75, 5.5, 17.5, 55, 175, 550, 2000 (or 1.75, 5.5, 17.5, 55, 175, 550,
1750, 5000 for specific regulatory needs).

20. 1 animal
1.75mg/kg
If the animal
survives
If the animal dies
Next animal receives the higher dose
5.5mg/kg
Classify GHS
category 1
1 animal
5.5mg/kg
If the animal survives If the animal dies
Next animal receives the higher dose
17.5mg/kg
Animal receives lower
dose

21.  The testing stops when one of the following stopping criteria first is met:
(a)3 consecutive animals survive at the upper bound;
(b)5 reversals occur in any 6 consecutive animals tested;
(c)at least 4 animals have followed the first reversal and the specified
likelihood-ratios exceed the critical value.
• If an animal unexpectedly dies late in the study and there are other survivors
at that dose or above, it is appropriate to stop dosing and observe all animals
to see if other animals will also die during a similar observation period.
• If subsequent survivors also die, and it appears that all dose levels exceed the
LD50 it would be most appropriate to start the study again.
• If subsequent animals survive at or above the dose of the animal that dies, it
is not
necessary to change the dose progression.

22. OBSERVATIONS
• Animals are observed individually at least once during the first 30 minutes after dosing, periodically
during the first 24 hours for 14 days.
• The times at which signs of toxicity appear and disappear are recorded.
• Observations should include
- changes in skin and fur
- eyes and mucous membranes
- respiratory, circulatory
- autonomic and central nervous systems
- somatomotor activity
- behaviour pattern
- tremors, convulsions
- salivation
- diarrhoea
- lethargy
- sleep and coma.
• Animals found in a moribund condition and animals showing severe pain or enduring signs of severe
distress should be humanely killed and time of death should be recorded.

23. Bodyweight
• Individual weights are recorded before the test drug administration and
weekly thereafter.
• At the end of the test surviving animals are weighed and then humanely
killed.
Pathology
• All animals (including those which die during the test or are removed
from the
study for animal welfare reasons) should be subjected to gross necropsy.
• Microscopic examination of organs showing evidence of gross
pathology in animals surviving 24 or more hours after the initial
dosing may also be considered.

24. DATA AND REPORTING
All data should be summarized in tabular form, showing for each test dose,
-number of animals used
-number of animals displaying signs of toxicity
-number of animals found dead during the test or killed for humane reasons
-time of death of individual animals
-a description and the time course of toxic effects and reversibility
-necropsy findings.
CALCULATION OF LD 50
The LD50 is calculated using the maximum likelihood method Likelihood
function is written as follows:
L = L1 L2 ....Ln ,
where L is the likelihood of the experimental outcome, µ and σ, and n the total number of
animals tested.

25. Li = 1 - F(Zi) ,if the ith animal survived,
or Li = F(Zi) if the ith animal died,
where F = cumulative standard normal distribution,
Zi = [log(di) - µ ] / σ
di = dose given to the ith animal
σ = standard deviation in log units of dose (usually 0.5)
Estimated LD50 are available for these circumstances given below:
a) If the upper bound dose ended testing, then the LD50 is reported to be above the upper
bound.
b) If all the dead animals have higher doses than all the live animals, then the LD50 is
between the doses for the live and the dead animals which do not give exact value of
LD50.
c) If the live and dead animals have only one dose in common and all the other dead
animals
have higher doses and all the other live animals lower doses, or vice versa, then the LD50
equals their common dose.
 Maximum likelihood calculation can be performed using either SAS (e.g., PROC NLIN) or
BMDP (e.g., program AR) computer program packages.

26. Computation of Confidence Interval
Confidence intervals provides valuable information on the reliability and utility of
the main test that was conducted
• A wide confidence interval indicates that there is more uncertainty
associated with the estimated LD50. The reliability of the estimated LD50 is
low and the usefulness of the estimated LD50 may be marginal.
• A narrow interval indicates that there is relatively little uncertainty associated
with the estimated LD50. The reliability of the estimated LD50 is high and the
usefulness of the estimated LD50 is good.
• This means that if the main test were to be repeated, the new estimated LD50
should be close to the original estimated LD50 and both of these estimates
should be close to the true LD50.

27. TEST REPORT
The test report must include the following information:
• Test substance:
–physical nature, purity and, where relevant, physical-chemical properties
(including isomerization);
–identification data, including CAS number.
• Vehicle (if appropriate):
–justification for choice of vehicle, if other than water.
• Test animals:
–species/strain used;
–microbiological status of the animals, when known;
–number, age and sex of animals (including, where appropriate, a rationale for use
of males instead of females);
–source, housing conditions, diet, etc.

28. • Test conditions:
– rationale for initial dose level selection, dose progression factor and for follow-up dose
levels;
– details of test substance formulation including details of the physical form of the material
administered;
– details of the administration of the test substance including dosing volumes and time of
dosing;
– details of food and water quality (including diet type/source, water source).
Results:
– body weight/body weight changes;
– tabulation of response data and dose level for each animal (i.e., animals showing signs of
toxicity including nature, severity, duration of effects, and mortality);
– individual weights of animals at the day of dosing, in weekly intervals thereafter, and at
the time of death or sacrifice ;
– time course of onset of signs of toxicity and whether these were reversible for each animal;
– necropsy findings and any histopathological findings for each animal, if available; − LD50
data;
– statistical treatment of results.

29. Comparison of guidelines
Criteria 420 423 425
Dose 5 to 5000 mg/kg 5 to 5000 mg/kg 1st animal dose less than
estimated LD50
outcome by factor 3.2
Grouping of animal Group of 5 animal of
one sex
Three animals per
group
One animal per group
Estimation of end
point
1 animal expected to
die
2-3 animal
expected to die
2-3 animal expected to
die
Sighting study Present Absent Absent